In recent years, in order to increase radio communication rates and capacities in radio communication systems such as mobile phone systems (cellular systems), a next-generation radio communication technique has been discussed. For example, in the 3rd Generation Partnership Project (3GPP) that is a standards body, the communication standard that is referred to as Long Term Evolution (LTE) and the communication standard that is referred to as LTE-Advanced (LTE-A) based on the radio communication technique for LTE have been proposed.
The latest communication standard completed by the 3GPP is Release 10 supporting LTE-A. Release 10 is the communication standard completed by significantly expanding the functions of Release 8 and 9 supporting LTE. The discussion of main parts of Release 11 to be completed by expanding Release 10 was concluded and Release 11 is under discussion for the completion of Release 11. In addition, the discussion of Release 12 has been started. Hereinafter, unless otherwise noted, “LTE” includes LTE, LTE-A, and other radio communication systems obtained by expanding LTE and LTE-A.
For LTE and LTE-A, many various techniques have been proposed, discussions have been held based on the proposals, and the communication standards will be expanded. In the discussions, many techniques were not adopted for some reasons regardless of having been proposed, and many techniques were decided to be discussed in the future based on relationships between priorities.
One of the techniques is multi-subframe scheduling. Since multi-subframe scheduling is a form of cross-subframe scheduling, cross-subframe scheduling is described below.
In downlink data communication (in a direction from a radio base station to a radio terminal) in a general LTE system, a subframe in which data is transmitted is the same as a subframe in which control information accompanied with the data and provided for scheduling or the like is transmitted. On the other hand, if cross-subframe scheduling is introduced, a subframe in which data is transmitted may be different from a subframe in which control information accompanied with the data and provided for scheduling or the like is transmitted. Each of the subframes is obtained by dividing a radio frame including a frequency and a time by a time period. In the LTE system, each of the subframes has a length of 1 millisecond.
Since cross-subframe scheduling is not limited to a constraint in which control information and data are transmitted in the same subframe in a conventional technique, flexible scheduling may be achieved. For example, a region (radio resource) in which control information may be set and that is included in a subframe is limited in the LTE system. If a large number of data items are to be transmitted (for example, if a large number of data items with small amounts are to be transmitted), a region for a control signal may not be available. If it is expected that a large number of data items are to be transmitted, and a part of control information for the data items is transmitted in a subframe that is different from the data items, it is possible to transmit the larger number of data items and avoid the fact that the region for the control signal is not available.
Next, the aforementioned multi-subframe scheduling is described. If multi-subframe scheduling is introduced, one or multiple data items to be transmitted in multiple subframes may be controlled based on a single control information item. A subframe in which control information is transmitted may be any of subframes in which data is transmitted or may be different from the subframes in which the data is transmitted. In both cases, in multi-subframe scheduling, at least one of the subframes in which the data is transmitted is different from the subframe in which the control information is transmitted, and cross-subframe scheduling is inevitably executed. Thus, it may be said that multi-subframe scheduling is a form of cross-subframe scheduling.
Multi-subframe scheduling provides an effect obtained by cross-subframe scheduling and an effect of reducing the amount of a control signal. The reason is that, in multi-subframe scheduling, a single control signal may be used for one or multiple data items in multiple subframes. As described above, the region for the control signal is limited. Thus, a reduction in the amount of the control signal is considered as an important issue. In addition, if the amount of the control signal is large, the number of radio resources for data transmission is reduced and it is difficult to achieve a high throughput. Thus, it may be said that there has been an increasing demand to reduce the amount of the control signal in recent years.
On the other hand, in LTE-A, a technique that is referred to as carrier aggregation is introduced. Carrier aggregation is a technique for transmitting and receiving multiple carriers (frequency bands) in parallel. Carrier aggregation is known as one of element techniques that achieve a high throughput in radio communication. For example, if two carriers of the same frequency bandwidth are used in parallel, the throughput may be doubled, compared with a case where one carrier is used.
Some scheduling methods for carrier aggregation are known, and one of the scheduling methods is multi-carrier scheduling. Multi-carrier scheduling is a form of cross-carrier scheduling. Thus, cross-carrier scheduling is described below.
If carrier aggregation is executed, data and a control signal accompanied with the data and provided for scheduling or the like may be transmitted on the same carrier for each of multiple carriers. On the other hand, a method of using a single carrier selected from among multiple carriers to transmit control information for data items that are each to be transmitted on any of the carriers has been proposed. The method in which a carrier to be used to transmit data is different from a carrier to be used to transmit a control signal accompanied with the data and provided for scheduling or the like is referred to as cross-carrier scheduling. A radio terminal monitors a downlink control signal in order to detect the transmission of data whose destination is the interested radio terminal. Thus, if the control signal is transmitted on multiple carriers, a load applied due to the monitoring is large and the monitoring is inconvenient. In order to avoid this problem with carrier aggregation, it is considered to be effective to set the number of carriers to be monitored by cross-carrier scheduling to one or reduce the number of carriers to be monitored by cross-carrier scheduling.
Next, the aforementioned multi-carrier scheduling is described. If multi-carrier scheduling is introduced, one or multiple data items to be transmitted on multiple carriers may be controlled by one control information item. A carrier for transmitting the control information item may be any of the carriers for transmitting the data items or may be a carrier different from the carriers for transmitting the data items. In both cases, in multi-carrier scheduling, since at least one of the carriers for transmitting the data items is different from the carrier for transmitting the control information item, cross-carrier scheduling is inevitably executed. Thus, it may be said that multi-carrier scheduling is a form of cross-carrier scheduling.
Multi-carrier scheduling provides an effect obtained by cross-carrier scheduling and an effect of reducing the amount of a control signal. The reason is that, in multi-carrier scheduling, a single control signal may be used for one or multiple data items on multiple carriers. As described above, the region for the control signal is limited. Thus, a reduction in the amount of the control signal is considered as an important issue. In addition, if the amount of the control signal is large, the number of radio resources for data transmission is reduced and it is difficult to achieve a high throughput. Thus, it may be said that there has been an increasing demand to reduce the amount of the control signal in recent years.
Multi-subframe scheduling and multi-carrier scheduling are described above. It may be said that multi-subframe scheduling and multi-carrier scheduling are scheduling methods using only one control signal for scheduling or the like for one or multiple data items to be transmitted by multiple radio resources (subframes and carriers).
In this specification, multi-subframe scheduling and multi-carrier scheduling are collectively referred to as “multi-subframe scheduling and the like” in some cases. In addition, multiple subframes and multiple carriers are collectively referred to as “multiple subframes and the like” in some cases. Since subframes and carriers are radio resources, a subframe and a carrier are collectively referred to as a “radio resource” in some cases, and multiple subframes and multiple carriers are collectively referred to as “multiple radio resources” in some cases. A single subframe of a single carrier is a unit of a radio resource in multi-subframe scheduling and the like and is thus referred to as a unit radio source in some cases.
Examples of related art are Japanese Laid-open Patent Publications Nos. 2012-60539, 2012-165439, 2012-130070, 2012-138968, and 2012-5074, Japanese National Publication of International Patent Application No. 2011-517889, U.S. Patent Application Publications Nos. 2012/0127938, 2010/0309867, 2011/0064037, 2011/0105050, 2011/0274060, and 2011/0274064, and Non-Patent Documents “3GPP TS36. 212 V11.1.0 (2012-12-20)” and “3GPP TS36. 213 V11.1.0 (2012-12-20)”.